The Question:

I just cracked an egg and it had two yolks. How does this work?
What are the odds of this happening?

The Answer:

This happens when two yolks are dropped from the ovary at the
same time. Because they are so close as they go through the oviduct,
they are treated as one yolk and only one shell is made. Double-yolk
eggs are usually produced by young chickens whose laying cycle has not
yet been established, but some chickens are genetically more likely to
lay them regularly. Sources we've found online differ considerably as
to the odds, ranging from 0.1% to 5%.